The present invention relates to a method for positioning a skewed frame in a microfiche retrieval device.
As is well-known, when automatically retrieving a frame recorded on a microfiche with a microfiche retrieval device, the target frame is automatically positioned by inputting the address of the frame (expressed by the numbers of frames from the reference frame in rows and columns) from a keyboard of the retrieval device to make the device convey the microfiche and to detect a frame corresponding to the address so as to be able to project the target frame on a reader screen.
FIG. 1 graphically shows an example of a frame arrangement of a microfiche which has hitherto been used. On this microfiche are recorded (m+1) frames along the horizontal (row) direction X and (n+1) frames along the vertical (column) direction Y from a "reference frame" P.sub.o (its address being (0,0)) which is the first address at the top left corner to a "span frame" P.sub.s (its address being (m, n)) which is the largest address at the right bottom corner in the figure. The pitch between adjacent frames is known since it has been specified when photographed. The value is A.sub.x in the direction of the X axis and A.sub.y in the direction of Y axis. The value is presumed to be manually inputted into the retrieval device in advance.
Conventionally, the reference frame P.sub.o which is the reference point for positioning frames is first set at the position of projection of the retrieval device. The device measures the distance from the mechanical origin (not shown) thereof by the amount of displacement (for example, the number of pulses) when the film is conveyed and calculates the position of the reference frame P.sub.o in terms of coordinate value (X.sub.o,Y.sub.o). When the address of the frame to be retrieved, for example, (i,j) of ".alpha." is inputted, the coordinate value (X.sub.i,Y.sub.j) of the frame ".alpha." is calculated from the following formula (1) using the coordinate value (X.sub.o,Y.sub.o) of the reference frame P.sub.o and the reference pitches A.sub.x and A.sub.y. ##EQU1##
Such a positioning method is only applicable to a microfiche whose frames are arranged in parallel with the directions in which the retrieval device moves (along X and Y axes). In practice, however, the arrangement of the frames is not necessarily in parallel with the moving axes of the retrieval device due to deviation of the film from the retrieval device when photographed, etc. This brings up a problem that even if the above mentioned positioning is carried out, the object frame cannot be accurately positioned on the reader screen and the position has to be further adjusted manually each time.
A method for positioning a frame which solves such problems is disclosed in the specification of U.S. Pat. No. 4,287,564. According to the disclosed method, as shown in FIG. 2 which corresponds to FIG. 1, the arrangement of frames slants against the axes (X and Y axes) of the retrieval device. It is assumed that the number {(m+1).times.(n+1) } of frames is known. As explained above, the position of the reference frame P.sub.o is measured in terms of the coordinates (X.sub.o,X.sub.o) while the position of the frame P.sub.M which as the maximum address along the same row as the reference frame P.sub.o (the address thereof (m,0)) is measured in terms of coordinate values (X.sub.m,Y.sub.o '). The displacement L.sub.x therebetween in the direction of the X axis and the deviation .DELTA..sub.y in the direction of the Y axis due to skew are calculated from the coordinate values using the following formula (2). ##EQU2##
Using the displacement L.sub.x, the deviation .DELTA..sub.y in the direction Y axis and the number m of frames therebetween, the reference pitch A.sub.x ', and a reference correction .delta..sub.y per frame are obtained from the following formula (3). ##EQU3##
Similarly, the position of a frame P.sub.N (its address being (0,n)) which has the maximum address in the same column as the reference frame P.sub.o is measured in terms of coordinate value (X.sub.o ',Y.sub.n). The displacement L.sub.y therebetween along the direction of Y axis and the deviation .DELTA..sub.y due to skew are calculated from the coordinate values using the following formula (4). ##EQU4##
Using the displacement L.sub.y, the deviation .DELTA..sub.x along X axis and the number n of frames therebetween, the reference pitch A.sub.y, and the reference correction .delta..sub.x per frame are calculated from the following formula (5). ##EQU5##
The reference correction .delta..sub.x along the direction of X axis is the amount of deviation between frames along the same row (the first row in this example) while the reference correction .delta..sub.y along the direction of Y axis is the amount of deviation between frames in the same column (the first column in this example). Thus, the reference correction .delta..sub.y in the direction of Y axis is accumulated at the row address while the reference correction .delta..sub.x in the direction of X axis at the column address, both of which increase respectively.
If the frame address of a frame to be retrieved, the frame address (i,j) of a frame ".SIGMA." as shown in FIG. 2 for example is inputted, the coordinate value (X.sub.i,Y.sub.j) of the frame .SIGMA. can be calculated using the coordinate values (X.sub.o,Y.sub.o) of the reference frame P.sub.o and the reference pitches A.sub.x ' and A.sub.y ' obtained from the above mentioned formulas (3) and (5) and reference corrections .delta..sub.x and .delta..sub.y by the following formula (6). ##EQU6##
According to the method for positioning a frame, as explained above, the maximum frame distances L.sub.x and L.sub.y in both the row and the columns of the microfiche are measured, which are then respectively divided by the numbers m and n of frames therebetween to obtain the reference pitches A.sub.x and A.sub.y '. At the same time, the reference corrections .delta..sub.x and .delta..sub.y are obtained from the deviations .DELTA..sub.x and .DELTA..sub.y caused by slanting and corrections are made to the above reference pitches A.sub.x ' and A.sub.y '. Even if theframe images are slanted on a microfiche, the skew is corrected to position the object frame accurately.
In such a method for positioning a frame, the reference frame P.sub.o and the maximum address frames P.sub.M and P.sub.N of the row and the column have to be photographed. Furthermore, such frame images have to be very clear as they play the role of the reference. With some microfiches, however, only a letter for an index is recorded in the reference frame P.sub.o, for example, or recorded on the maximum address frames P.sub.M and P.sub.N for the convenience of editing. Prior art method for positioning a frame as explained above was not applicable to such microfiches.